The present invention relates to devices for monitoring the quality of silk-screen printing on plastic or glass bottles of any shape.
In the known art this monitoring is effected by video cameras arranged to photograph individual images of the object to be inspected in successive positions, and compare them with a like number of reference images to determine any discrepancies.
In particular, the known devices for monitoring print quality on bottles of circular cross-section comprise a number of successive inspection stations, in each of which a video camera photographs the image of the bottle to be inspected in a determined position.
If the device comprises two stations the bottle is rotated through 180.degree. prior to the second station, so that by placing the two acquired images side by side an image of the entire bottle surface is reconstructed, this then being compared with the reference image.
If there are three image inspection stations, the bottle has to be rotated through 120.degree. prior to each station following the first.
It is also known that the photographic image of a circular or otherwise non-flat object presents distortions commencing from the centre plane. These distortions are greater the smaller the radius of curvature of the object surface, and the larger the lens opening angle, for equal distances between the object centre and the lens. Consequently the known devices have the drawback of lack of photographic accuracy in proximity to the lateral portions of the image.
In the known art this drawback is reduced by providing at least four inspection stations each comprising its own video camera, and means for rotating the object through 90.degree. between one station and the next, but this in addition to being complicated is also of high cost.